I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a method and apparatus for supervising a control channel used in a telecommunications system.
II. Description of the Related Art
The telecommunications Industry Association developed a standard for code division multiple access (CDMA) communications systems in the Interim Standard IS-95A, entitled xe2x80x9cMobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d (hereafter IS-95). In IS-95 systems, the mobile station controls the energy of its transmissions by means of a combination of open loop and closed loop power control methods. In open loop power control, a mobile station measures the received energy of the forward link signal from a serving base station and adjusts the energy of its reverse link transmission in accordance with this measurement. In closed loop power control, the serving base station measures the energy of transmissions from the mobile station and sends a series of up/down commands based on this measurement to the mobile station which adjusts its transmissions in response. A power control system employing closed loop and open loop power control is described in U.S. Pat. No. 5,056,109, entitled xe2x80x9cMETHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein.
In IS-95, the mobile station is required to monitor the Forward Traffic Channel performance during a call. When the mobile station receives twelve (N2m) consecutive bad frames, the mobile station is required to disable its transmitter so that it will not jam the reverse link. Thereafter, if the mobile station receives two (N3m) consecutive good frames, it should re-enable its transmitter. The mobile station also maintains a fade timer. The fade timer is first enabled when the mobile station enables its transmitter at the beginning of a call, and it is reset for five (T5m) seconds whenever two (N3m) consecutive good frames are received on the Forward Traffic Channel. If the fade timer expires, the mobile station disables its transmitter and declares a loss of the Forward Traffic Channel and terminates the call.
The International Telecommunications Union recently requested the submission of proposed methods for providing high rate data and high-quality speech services over wireless communication channels. A first of these proposals was issued by the Telecommunications Industry Association, entitled xe2x80x9cThe cdma2000 ITU-R RTT Candidate Submissionxe2x80x9d (hereafter cdma2000). In cdma2000, the equivalents of the Forward Traffic Channel in IS-95 are the Forward Fundamental Channel (F-FCH) and the Forward Dedicated Control Channel (F-DCCH). The data frames transmitted on these channels can be either 20 ms or 5 ms in duration. For F-FCH, a frame (20 or 5 ms) is transmitted in every 20 ms interval aligned to the beginning of the CDMA System Time. For F-DCCH, the transmission can be discontinuous, such that there may not be any data frame transmitted in a 20 ms interval aligned to the CDMA System Time.
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation technique of CDMA has significant advantages over these modulation techniques for multiple access communication systems. The use of CDMA techniques in multiple access communication systems is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,xe2x80x9d and U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d both of which are assigned to the assignee of the present invention and incorporated by reference herein.
CDMA by its inherent nature of being a wideband signal offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled xe2x80x9cMETHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d and U.S. Pat. No. 5,109,390 entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d both assigned to the assignee of the present invention and incorporated by reference herein.
In a communication system that provides data using a Quadrature Phase-Shift Keying (QPSK) modulation format, useful information can be obtained by taking the cross product of the I and Q components of the QPSK signal. By knowing the relative phases of the two components, one can determine roughly the velocity of the mobile station in relation to the base station. A description of a circuit for determining the cross product of the I and Q components in a QPSK modulation communication system is disclosed in U.S. Pat. No. 5,506,865, entitled xe2x80x9cPILOT CARRIER DOT PRODUCT CIRCUIT,xe2x80x9d assigned to the assignee of the present invention, the disclosure of which is incorporated by reference herein.
There has been an increasing demand for wireless communications systems to be able to transmit digital information at high rates. One method for sending high rate digital data from a remote station to a central base station is to allow the remote station to send the data using spread spectrum techniques of CDMA, such as that proposed in U.S. Pat. No. 6,396,804, entitled xe2x80x9cHIGH DATA RATE CDMA WIRELESS COMMUNICATION SYSTEM,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein.
New methods for supervising the F-DCCH are needed when F-DCCH is in this discontinuous transmission (DTX) mode because the mobile station must now decide whether a received frame is a good frame, a bad frame, or an empty frame (i.e., no transmission), and how to handle the transmission based upon the type of frames received.
The present invention is a method and apparatus for supervising a dedicated control channel used in a wireless communication system. The invention solves a variety of F-DCCH supervision problems when an empty frame is encountered.
A first embodiment is an improvement upon the methods used in IS-95, where empty frames are simply ignored. The mobile station maintains a counter of consecutive bad frames, COUNT1 (CNT1), a counter of consecutive good frames, COUNTs (CNTs) is set to zero at the beginning of a call. For each frame received, the A mobile station determines if it is a good frame, a bad frame, or an empty frame. If the (received frame is a good frame, CNT1 and CNT3 are reset to zero and CNT2 is incremented by 1. If the received frame is a bad frame, CNT1 is incremented by one and CNT2 is reset to zero. If CNT1 reaches a first threshold value, TH1, the mobile station disables its transmitter. If CNT2 reaches a second threshold value, TH2, the mobile station re-enables its transmitter. The mobile station resets its fade timer to X seconds whenever CNT2 reaches a third threshold value, TH3.
If the received frame is an empty frame, CNT1 and CNT2 are unchanged, but CNT3 is incremented by one. If CNT3 reaches a fourth threshold value, TH4, the mobile station disables its transmitter. If CNT3 reaches a fifth threshold value, TH5, the mobile station disables its transmitter, if not already disabled, and declares the Forward Traffic Channel as lost.
In another embodiment, the mobile station uses the received pilot strength (Ec/ho) of pilots in the Active Set to perform F-DCCH supervision. The method aggregates the Ec/ho of all pilots in the Active Set and averages them over a designated time interval. If this average aggregated value (AAV) is below a threshold for a specified amount of time, then the mobile station disables its transmitter. If the AAV continues below the threshold for a longer specified period of time, then the mobile station disables its transmitter, if not already disabled, and declares the Forward Traffic Channel as lost.
As readily recognizable to one skilled in the art, the invention also provides a number of advantages and benefits that will become apparent after reviewing the following description of the invention.